Heretofore, thermoplastic materials such as polystyrene or polyethylene foam materials have been trimmed by heat-cutting with a thin tool. This creates a ridge or flash on the formed side of the material, which must be removed in a separate trimming operation. It also leaves a residue on the knife which cannot be heated to a temperature high enough to vaporize the material and still provide a clean edge. This residue must frequently be removed in order that the knife may perform an effective trimming operation.
Examples of prior art apparatus and processes are in the U.S. Pat. Nos. to Haase 3,684,633; Seto 3,475,526; Langdon et al 3,362,043; and Scalora 3,240,851.
In accordance with the principles of the present invention, the flash or ridge formed by the cutting tool, when cutting or trimming plastic material, is removed by pressurizing the material along the edge to be trimmed by a trim guide in the form of two pieces of metal of high heat-conductivity, which may be die or mold parts or separate parts of high heat-conductivity between which the material is placed. These two parts made from metal of high heat-conductivity are then moved toward each other to squeeze or pressurize the material along the area to be trimmed.
The plastic material may be a polystyrene or polyethylene foam of various densities, although it need not necessarily be a foam material, and where a sheen is required, may have a lamina of high-density polystyrene film on one side thereof which may be an oriented thermoplastic film.
A knife heated to a temperature high enough to melt the thermoplastic material, which may be high enough to vaporize the material, is then used to trim the material along the trim guie and provide a cleanly trimmed edge, which may be an internal or external edge, with no flash or heat ridge.
The trim guide is preferably made from aluminum, but may be made from other metals having high heat-conductivity, and holds and presses or pressurizes the material along the area to be trimmed and adsorbs the heat of cutting so the heat ridge or flash formerly present in heat-trimming of foam materials has no place to form. The trim guide may be the parts of a mold or die, and may be formed to accommodate a heated knife to trim the external periphery of a formed article or may be formed to accommodate a heated knife or tool to place slots or apertures of various forms in foam materials by pressurizing the material along the portion to be slotted or apertured while the heated knife or tool performs the slotting or aperturing operation, the trim guide or die parts of high heat-conductivity leaving no place for the flash or heat ridge to form during the slotting or aperturing operation.
The advantages of the present invention are that the deficiencies of heat-trimming of thermoplastic materials, such as polystyrene or polyethylene foam of the prior art are obviated by pressurizing a piece of thermoplastic material along the edge or perimeter to be trimmed by two pieces of metal of high heat-conductivity, and trimming the material along the two pieces of metal by a hot knife, hot enough to vaporize the material along the pieces of metal, which pieces of metal form a heat barrier at which the melting of the material stops.
Another advantage of the invention is that the heat ridge heretofore formed by trimming thermoplastic materials and particularly foam materials with a hot knife is obviated by the utilization of a knife or tool hot enough to melt the material and moving this knife or tool during its cutting operation along surfaces of high heat-conductivity, pressurizing the edge of the material, and cold enough to provide a heat barrier.
A still further advantage is that by confining foam material between two pieces of metal of high heat-conductivity and exerting pressure on the material along the edge or perimeter to be trimmed, a definitive point is created at which melting by the heated knife or trimming tool stops.
Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure.